1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic method, such as a laser printer, a copying machine, and a facsimile machine.
2. Description of the Related Art
In some conventional image forming apparatuses of the electrophotographic method, process units, such as a photosensitive drum and a developing unit, in each image forming unit are integrated into a process cartridge, and such process cartridges are arranged in line to be attachable to and detachable from the image forming apparatus. This arrangement enables the user, without the aid of a service engineer, to replace process cartridges, for example, in the event of run-out of developer as well as to replace other consumables, such as photosensitive drums, at the same time, thus enhancing maintenance performance.
As a developing method employed in process cartridges for use in such image forming apparatuses, a contact developing method is generally widely adopted in which development is performed with a developing roller, serving as a developing unit, kept in contact with a photosensitive drum.
In image forming apparatuses using the contact developing method, if a developing roller and a photosensitive drum are not used for a long time with them kept in contact with each other, an elastic layer of the developing roller may be deformed, or developer borne on the developing roller may unnecessarily adhere to the photosensitive drum, thus causing image defects.
To address this issue, Japanese Patent Application Laid-Open No. 2007-213024 discusses an image forming apparatus equipped with a separation mechanism that acts on a process cartridge, when image formation is not performed, to separate a developing roller from a photosensitive drum. The separation mechanism for the developing roller arranged in the image forming apparatus takes three contact/separation states in which, in each image forming unit, a developing roller is in contact with or separated from a photosensitive drum. More specifically, the three contact/separation states include a “full-color image forming state” in which, in all of the image forming units, the developing roller is in contact with the photosensitive drum, a “mono-color image forming state” in which, in only the black image forming unit, the developing roller is in contact with the photosensitive drum, and a “standby state” in which, in all of the image forming units, the developing roller is separated from the photosensitive drum.
The image forming apparatus discussed in Japanese Patent Application Laid-Open No. 2007-213024 changes over between a contact state and a separation state of the developing roller and the photosensitive drum according to an operation of the separation mechanism of the image forming apparatus.
The state of the separation mechanism changes in order, such as “standby state”→“full-color image forming state”→“mono-color image forming state”→“standby state”→ . . . .
The separation mechanism, when in the standby state, separates the developing roller from the photosensitive drum in all of the image forming units. Thus, the standby state is a state taken when the image forming apparatus is on standby without performing image formation. Also, the separation mechanism, when in the full-color image forming state, cancels the separation state and brings the developing roller into contact with the photosensitive drum in all of the image forming units. Thus, the full-color image forming state is a state taken when the image forming apparatus forms a full-color image. Furthermore, the separation mechanism, when in the mono-color image forming state, cancels the separation state and brings the developing roller into contact with the photosensitive drum in only the image forming unit that forms a black image. On the other hand, in the other forming units (yellow, cyan, and magenta image forming units), the separation mechanism, when in the mono-color image forming state, separates the developing roller from the photosensitive drum. Thus, the mono-color image forming state is a state taken when the image forming apparatus forms a mono-color (black and white) image.
Then, after the completion of a normal image forming operation, the separation mechanism separates all of the developing rollers from the respective photosensitive drums to enter the “standby state” and, then, the image forming apparatus terminates the entire operation.
In this instance, due to the attachment or detachment of a process cartridge by the user, the power on or off of the image forming apparatus, or the plugging or unplugging of the image forming apparatus, the state of the separation mechanism may be different from the actual contact/separation state of the photosensitive drum and the developing roller.
For example, a case can be considered where, when the separation mechanism of the image forming apparatus is in the “standby state”, a process cartridge is taken out of the main body of the image forming apparatus and the process cartridge is then inserted into the main body with the developing roller and the photosensitive drum kept in contact with each other. In such a case, the state of the separation mechanism is different from the actual contact/separation state of the photosensitive drum and the developing roller. In other words, while the state of the separation mechanism is in the standby state (originally, a state to separate the developing roller from the photosensitive drum), the developing roller is actually in contact with the photosensitive drum.
If the state of the separation mechanism does not coincide with the actual contact/separation state of the photosensitive drum and the developing roller, the image forming apparatus becomes unable to recognize the actual contact/separation state of the photosensitive drum and the developing roller (the actual contact/separation state of the photosensitive drum and the developing roller becomes unrecognizable). In such a situation, the image forming apparatus may not be able to correctly control the contact/separation state of the photosensitive drum and the developing roller.
Accordingly, in a case where the actual contact/separation state of the photosensitive drum and the developing roller becomes unrecognizable, the image forming apparatus is required to conform the state of the separation mechanism to the actual contact/separation state of the photosensitive drum and the developing roller before performing an image forming operation.
Therefore, the image forming apparatus performs, in an initial operation (a preparation operation prior to an image forming operation), control to cause the separation mechanism to operate to shift from the “standby state” to the “full-color image forming state”.
Thus, before the initial operation (for example, immediate after the image forming apparatus is powered on), there is a possibility that the state of the separation mechanism is different from the actual contact/separation state of the photosensitive drum and the developing roller.
Therefore, if the separation mechanism is shifted from the standby state to the full-color image forming state due to the initial operation, the developing roller is brought into contact with the photosensitive drum after the initial operation in all of the image forming units irrespective of the contact/separation state taken before the initial operation.
Thus, if the separation mechanism is shifted, in the initial operation, to the “full-color image forming state” to bring all of the developing rollers into contact with the respective photosensitive drums, the state of the separation mechanism coincides with the actual contact/separation state of the photosensitive drum and the developing roller. Once the state of the separation mechanism coincides with the actual contact/separation state of the photosensitive drum and the developing roller, even when the separation mechanism is caused to operate after the initial operation, the state of the separation mechanism constantly becomes coincident with the actual contact/separation state of the photosensitive drum and the developing roller. The image forming apparatus is thus able to surely control the contact/separation state.
However, in a case where the above-described control is performed, the time required for the initial operation lengthens by a time required for the separation mechanism to operate in the initial operation, so that the time at which the image forming operation starts would become late.
For example, an initial operation after the user has performed detachment and attachment of process cartridges when the separation mechanism of the image forming apparatus is in the “standby state” is described. Here, the initial operation is an operation initially performed to check whether any residual paper remains inside the image forming apparatus or the image forming apparatus functions well after the image forming apparatus is powered off and on or after the paper jam is removed.
FIG. 25 is a timing chart illustrating the initial operation performed in the above-mentioned case. The initial operation illustrated in FIG. 25 is a calibration operation. The calibration operation is an operation to adjust the tint or color misregistration of an image output from the image forming apparatus. In the calibration operation, the tint or color misregistration of the image is corrected by a method including forming a calibration patch image on a photosensitive drum, transferring the formed patch image onto an intermediate transfer belt, and detecting the patch image on the intermediate transfer belt.
Here, to form the calibration patch image in the initial operation, all of the developing rollers are required to be actually in contact with the respective photosensitive drums. Therefore, after starting driving of a main motor, the image forming apparatus first causes the separation mechanism to operate to shift from the “standby state” to the “full-color image forming state” (period “a” in FIG. 25). With this shift, all of the developing rollers are actually brought into contact with the respective photosensitive drums, so that, at this time, the state of the separation mechanism becomes coincident with the actual contact/separation state of the photosensitive drum and the developing roller. Then, the image forming apparatus transfers toner, which has adhered to each photosensitive drum during contact of each developing roller, onto the intermediate transfer belt, and cleans the intermediate transfer belt to remove the toner. To perform such transfer and cleaning, the image forming apparatus causes the separation mechanism to operate to shift from the “full-color image forming state” to the “mono-color image forming state” (period “b” in FIG. 25) and then from the “mono-color image forming state” to the “standby state” (period “c” in FIG. 25). In that state, the image forming apparatus performs cleaning of the intermediate transfer belt (period “θ1” in FIG. 25), and then causes the separation mechanism to operate again to shift from the “standby state” to the “full-color image forming state” (period “d” in FIG. 25). After that, the image forming apparatus performs an exposure operation to form a calibration patch image.
Then, after performing the calibration operation, the image forming apparatus cleans the intermediate transfer belt to remove the patch image. To perform such cleaning, the image forming apparatus causes the separation mechanism to operate again to shift from the “full-color image forming state” to the “mono-color image forming state” (period “e” in FIG. 25) and then from the “mono-color image forming state” to the “standby state” (period “f” in FIG. 25). In that state, the image forming apparatus performs cleaning of the intermediate transfer belt (period “θ2” in FIG. 25), and then stops the main motor to terminate the initial operation.
As mentioned above, prior to performing an exposure operation to form a calibration patch image, the image forming apparatus would require a waiting time by an operating time of the separation mechanism (period “a” to period “d” in FIG. 25) plus a cleaning time of the intermediate transfer belt (period “θ1” in FIG. 25). Therefore, it would be understood that the time required for the initial operation lengthens by the waiting time.
Furthermore, as the driving time for the initial operation increases, the rotation time of the main motor also increases, thus affecting the operating life of the main body of the image forming apparatus.